High thermal conductivity materials with grafted surface functional groups

ABSTRACT

The present invention is a continuous high thermal conductivity resin featuring high thermal conductivity (HTC) materials  30  and a host resin matrix  32 . The HTC materials  30  form a continuous organic-inorganic composite with the host resin matrix  32  via surface functional groups that are grafted to the HTC materials  30  and form covalent linkages with the host resin matrix  32 . Phonons  34  tend to pass along the HTC materials  30  as they travel through the host resin matrix  32 , and phonons  36  pass to the next HTC material if the distance between these materials is less than n.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 10/618,125(now U.S. Pat. No. 7,033,670), filed Jul. 11, 2003, and claims benefitto provisional application 60/580,023, filed Jun. 15, 2004, both ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The field of the invention relates to high thermal conductivitymaterials with surface grafted functional groups impregnated intoresins.

BACKGROUND OF THE INVENTION

With the use of any form of electrical appliance, there is a need toelectrically insulate conductors. With the push to continuously reducethe size and to streamline all electrical and electronic systems thereis a corresponding need to find better and more compact insulators andinsulation systems.

Various epoxy resin materials have been used extensively in electricalinsulation systems due to their practical benefit of being tough andflexible electrical insulation materials that can be easily adhered tosurfaces. Traditional electrical insulation materials, such as micaflake and glass fiber, can be surface coated and bonded with these epoxyresins, to produce composite materials with increased mechanicalstrength, chemical resistance and electrical insulating properties. Inmany cases epoxy resins have replaced traditional varnishes despite suchmaterials having continued use in some high voltage electricalequipment.

Good electrical insulators, by their very nature, also tend to be goodthermal insulators, which is undesirable. Thermal insulating behavior,particularly for air-cooled electrical equipment and components, reducesthe efficiency and durability of the components as well as the equipmentas a whole. It is desirable to produce electrical insulation systemshaving maximum electrical insulation and minimal thermal insulationcharacteristics.

Electrical insulation often appears in the form of insulating tapes,which themselves have various layers. Common to these types of tapes isa paper layer that is bonded at an interface to a fiber layer, bothlayers tending to be impregnated with a resin. A favored type ofinsulation material is a mica-tape. Improvements to mica tapes includecatalyzed mica tapes as taught in U.S. Pat. No. 6,103,882. The mica-tapemay be wound around conductors to provide extremely good electricalinsulation. An example of this is shown in FIG. 1. Illustrated here is acoil 13, comprising a plurality of turns of conductors 14, which in theexample illustrated here are assembled into a bakelized coil. The turninsulation 15 is prepared from a fibrous material, for example glass orglass and Dacron which is heat treated. Ground insulation for the coilis provided by wrapping one or more layers of composite mica tape 16about the bakelized coil 14. Such composite tape may be a paper or feltof small mica flakes combined with a pliable backing sheet 18 of, forexample, glass fiber cloth or polyethylene glycol terephthalate mat, thelayer of mica 20 being bonded thereto by a liquid resinous binder.Generally, a plurality of layers of the composite tape 16 are wrappedabout the coil depending upon voltage requirements. A wrapping of anouter tape 21 of a tough fibrous material, for example, glass fiber, maybe applied to the coil.

Generally, multiple layers of the mica tape 16 are wrapped about thecoil with sixteen or more layers generally being used for high voltagecoils. Resins are then impregnated into the tape layers. Resins can evenbe used as insulation independently from the insulating tape.Unfortunately this amount of insulation only further adds to thecomplications of dissipating heat. What is needed is electricalinsulation that can conduct heat higher than that of conventionalmethods, but that does not compromise the electrical insulation andother performance factors including mechanical and thermal capability.

Other difficulties with the prior art also exist, some of which will beapparent upon further reading.

SUMMARY OF THE INVENTION

With the foregoing in mind, methods and apparatuses consistent with thepresent invention, which inter alia facilitates the transport of phononsthrough a high thermal conductivity (HTC) impregnated medium to reducethe mean distances between the HTC materials below that of the phononmean free path length. This reduces the phonon scattering and produces agreater net flow or flux of phonons away from the heat source. Theresins may then be impregnated into a host matrix medium, such as amulti-layered insulating tape.

High Thermal Conductivity (HTC) organic-inorganic hybrid materials maybe formed from discrete two-phase organic-inorganic composites, fromorganic-inorganic continuous phase materials based on molecular alloysand from discrete organic-dendrimer composites in which theorganic-inorganic interface is non-discrete within the dendrimercore-shell structure. Continuous phase material structures may be formedwhich enhance phonon transport and reduce phonon scattering by ensuringthe length scales of the structural elements are shorter than orcommensurate with the phonon distribution responsible for thermaltransport, and/or that the number of phonon scattering centers arereduced such as by enhancing the overall structural order of the matrix,and/or by the effective elimination or reduction of interface phononscattering within the composite. Continuous organic-inorganic hybridsmay be formed by incorporating inorganic, organic or organic-inorganichybrid nano-particles in linear or cross-linked polymers (includingthermoplastics) and thermosetting resins in which nano-particlesdimensions are of the order of or less than the polymer or networksegmental length (typically 1 to 50 nm or greater). These various typesof nano-particles will contain reactive surfaces to form intimatecovalently bonded hybrid organic-inorganic homogeneous materials.Similar requirements exist for inorganic-organic dendrimers which may bereacted together or with matrix polymers or reactive resins to form acontinuous material. In the case of both discrete and non-discreteorganic-inorganic hybrids it is possible to use sol-gel chemistry toform a continuous molecular alloy. The resulting materials will exhibithigher thermal conductivity than conventional electrically insulatingmaterials and may be used as bonding resins in conventional mica-glasstape constructions, when utilized as unreacted vacuum-pressureimpregnation resins and as stand alone materials to fulfill electricalinsulation applications in rotating and static electrical power plantand in both high (approximately over 5 kV) and low voltage(approximately under 5 kV) electrical equipment, components andproducts.

The formation of engineered electrical insulation materials havingprescribed physical properties and performance characteristics, andbased on the use of nano-to-micro sized inorganic fillers in thepresence of organic host materials, requires the production of particlesurfaces which can form an intimate interface with the organic host.This may be achieved through the grafting of chemical groups onto thesurface of the fillers to make the surface chemically and physicallycompatible with the host matrix, or the surfaces may contain chemicallyreactive functional groups that react with the organic host to formcovalent bonds between the particle and the host. The use ofnano-to-micro sized inorganic fillers in the presence of organic hostmaterials requires the production of particles with defined surfacechemistry in addition to bulk dielectric and electrical properties andthermal conductivity. Most inorganic materials do not allow independentselection of structural characteristics such as shape and size andproperties to suit different electrical insulation applications or toachieve composites having the right balance of properties andperformance. This may be achieved by selecting particles withappropriate bulk properties and shape and size characteristics and thenmodifying the surface and interfacial properties and othercharacteristics to achieve the additional control of compositeproperties and performance required for electrical insulationapplications. This may be achieved by appropriate surface coating of theparticles which may include the production of metallic and non-metallicinorganic oxides, nitrides, carbides and mixed systems and organiccoatings including reactive surface groups capable of reacting withappropriate organic matrices which act as the host material in theelectrical insulation system. The resulting hybrid materials andcomposites in unreacted or partially reacted form may be used as bondingresins in mica-glass tape constructions, as unreacted vacuum-pressureimpregnation resins for conventional mica tape constructions, in otherglass fiber, carbon fiber and ply-type and textile composites and asstand alone materials to fulfill electrical insulation applications inrotating and static electrical power plant and in both high and lowvoltage electrical equipment, components and products.

In a particular embodiment the present invention provides for continuoushigh thermal conductivity resin that comprises a host resin matrix and ahigh thermal conductivity filler. The high thermal conductivity fillerforms a continuous organic-inorganic composite with the host resinmatrix via surface functional groups that are grafted to the highthermal conductivity filler and forms covalent linkages with the hostresin matrix.

In another particular embodiment the present invention provides forcontinuous organic-inorganic resin with grafted functional groupsbridging the organic-inorganic boundary that comprises a host resinnetwork and inorganic high thermal conductivity fillers evenly dispersedin the host resin network and essentially completely co-reacted with thehost resin network. The high thermal conductivity fillers have a lengthof between 1-1000 nm and aspect ratios of 10-50. The high thermalconductivity fillers are selected from one or more of oxides, nitrides,and carbides and the continuous organic-inorganic resin comprises amaximum of 60% by volume of the high thermal conductivity fillers, andin other embodiments a maximum of 35%. Particularly, the high thermalconductivity fillers have surface functional groups that are grafted tothe high thermal conductivity fillers and the surface functional groupsallow for the essentially complete co-reactivity with the host resinnetwork.

In related embodiments the functional groups comprise one or more ofhydroxyl, carboxylic, amine, epoxide, silane and vinyl groups. The oneor more of oxides, nitrides, and carbides comprise Al203, AlN, MgO, ZnO,BeO, BN, Si3N4, SiC and SiO2 with mixed stoichiometric andnon-stoichiometric combinations. The host resin network includes epoxy,polyimide, polyimide epoxy, liquid crystal epoxy, polybutadiene,polyester and cyanate-ester. The continuous organic-inorganic resinfurther can also comprises a cross-linking agent, and the entire resincan be impregnated into a porous media.

In still another particular embodiment the present invention providesfor method of making a high thermal conductivity resin that comprisessupplying a host resin matrix and gathering a high thermal conductivitymaterial, which is then surface treated with reactive surface functionalgroups in a high energy reaction such that the surface functional groupsbecome grafted to the high thermal conductivity materials. Then mixingthe treated high thermal conductivity materials with the host resinmatrix such that the high thermal conductivity materials aresubstantially uniformly dispersed within the host resin matrix, and thenreacting the surface functional groups that are grafted to the highthermal conductivity materials with the host resin matrix to produce thehigh thermal conductivity resin. The amount of the high thermalconductivity materials in the high thermal conductivity resin is amaximum of 60% by volume, and the high energy reaction produces bondstrength of between approximately 200-500 kJ/mol.

Other embodiments of the present invention also exist, which will beapparent upon further reading of the detailed description.

BRIEF DESCRIPTION OF THE FIGURES

The invention is explained in more detail by way of example withreference to the following drawings:

FIG. 1 shows the use of an insulating tape being lapped around a statorcoil.

FIG. 2 illustrates phonons traveling through a loaded resin of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

High thermal conductivity (HTC) composites comprise a resinous hostnetwork combined with fillers that are two phase organic-inorganichybrid materials. The organic-inorganic hybrid materials are formed fromtwo phase organic-inorganic composites, from organic-inorganiccontinuous phase materials that are based on molecular alloys, and fromdiscrete organic-dendrimer composites in which the organic-inorganicinterface is non-discrete with the dendrimer core-shell structure.Phonon transport is enhanced and phonon scattering is reduced byensuring the length scales of the structural elements are shorter thanor commensurate with the phonon distribution responsible for thermaltransport.

Two phase organic-inorganic hybrids may be formed by incorporatinginorganic micro, meso or nano-particles in linear or cross linkedpolymers (thermoplastics) and thermosetting resins. Host networksinclude polymers and other types of resins, definitions of which aregiven below. In general, the resin that acts as a host network may beany resin that is compatible with the particles and, if required, isable to react with the groups introduced at the surface of the filler.Nano-particle dimensions are typically of the order of or less than thepolymer network segmental length. For example 1-30 nm. The inorganicparticles contain reactive surfaces to form covalently bonded hybridorganic-inorganic homogeneous materials. The particles may be oxides,nitrides, carbides and hybrid stoichiometric and non-stoichiometricmixes of the oxides, nitrides and carbides, more examples of which aregiven below.

The inorganic particles are surface treated to introduce a variety ofsurface functional groups which are capable of participating inreactions with the host network. The surface functional groups includebut are not limited to hydroxyl, carboxylic, amine, epoxide, silane andvinyl groups. The groups may be applied using wet chemical methods,non-equilibrium plasma methods, chemical vapor and physical vapordeposition, sputter ion plating and electron and ion beam evaporationmethods.

The discrete organic-dendrimer composites may be reacted together orwith the resin matrix to form a single material. The surface of thedendrimer can contain reactive groups similar to those mentioned above,which will either allow dendrimer-dendrimer or dendrimer-organic matrixreactions to occur. The dendrimer will have an inorganic core and anorganic shell containing the reactive groups of interest. It may also bepossible to have an organic core with an inorganic shell which alsocontains reactive groups such as hydroxyl or silane groupings which canparticipate in inorganic reactions similar to those involved in commonsol-gel chemistries.

In regards to the use of non-discrete organic-inorganic hybrids it ispossible to use sol-gel chemistry to form a continuous molecular alloy.Gel sol-chemistries involving aqueous and non-aqueous reactions may beused. Other compounds for the formation of organic-inorganic hybridsinclude the polyhedral oligomeric silsesquioxanes (POSS), tetraethylorthosilicate (TEOS) and tetrabutyl orthotitanate (TBOT) and relatedmonomeric and oligomeric hybrid compounds which are organicfunctionalized inorganic compounds. In the example of POSS, moleculesare built around a building block of R—SiO_(1.5) in which the R group ischosen to compatibilize with and/or react with other organic compoundsand the host network. The base compounds may be combined to yield largermolecules commensurate with the size of polymer segment and coilstructures. POSS may be used to create organic-inorganic hybrids and maybe grafted into existing polymers and networks to control properties,including thermal conductivity. The materials may be obtained fromsuppliers such as Aldrich™ Chemical Co., Hybrid Plastics™ Inc. andGelest™ Inc.

As mentioned, it is important to control the structural form of thematerials to reduce phonon scattering. This can be further assisted byusing nano-particles whose matrices are known to exhibit high thermalconductivity and to ensure that the particles size and its interfacialcharacteristics with the resin are sufficient to sustain this effect,and also to satisfy the length scale requirement to reduce phononscattering. A choice of structures that are more highly ordered willalso benefit this, including reacted dendrimer lattices having bothshort and longer range periodicity and ladder or ordered networkstructures that may be formed from a host resin, such as liquid crystalepoxies and polybutadienes.

The filled resins may be used as bonding resins in a variety ofindustries such as circuit boards and insulating tapes. A particularkind of insulating tape is the mica-glass tape used in the electricalgenerator fields. Resins with these types of tapes can be used asbonding resins, or as impregnating resins as is known in the art. Thefilled resin may also be used in the electrical generator field withoutthe tapes to fulfill electrical insulation applications in the rotatingand static electrical equipment components.

The tapes may be impregnated with resin before or after being applied toelectrical objects. Resin impregnation techniques include VPI and GVPI,discussed more below. In VPI, once a tape is lapped and impregnated itis compressed. Once in position, the resin in the compressed tape iscured, which effectively locks the position of the HTC materials. Insome embodiments the resin is cured in a two stage process, as will beapparent to one of ordinary skill in the art. However, optimalcompression of the loaded HTC materials favors a completely uncuredresin during the compression stage.

FIG. 2 shows one embodiment of the present invention. Illustrated hereare HTC materials 30 loaded into a resinous matrix 32. Phonons 34traveling through the matrix have a mean path length n, this is thephonon mean free path. This path length can vary depending on the exactcomposition of the resin matrix, but is generally from 2 to 100 nm, andmore typically 5-50 nm, for resins such as epoxy resins. Therefore themean distance between the loaded HTC materials should be on average lessthan this distance. Note that the distance between the HTC materials canvary in the thickness versus transverse direction of the tape, and it isgenerally the thickness direction where the spacing needs to beoptimalized.

As phonons 34 travel through the resin 32 they will tend to pass alongthe embedded HTC materials 30. This will increase the local phonon fluxsince the raw HTC materials will have a thermal conductivity of between10-1000 W/mK, as opposed to the resin which is about 0.1-0.5 W/mK. Asphonons pass along a loaded HTC material the phonons 36 pass to the nextHTC material if the distance between the materials is less than n,therefore the HTC materials form an interconnecting network. FIG. 2illustrates an idealized path. In practice there will be phononscattering as the phonons pass between the resin and HTC materials,although the shorter the distance between the materials, and the betterthe match of phonon propagation characteristics between the HTCmaterials and the resin, the less the scattering.

The amount of HTC materials loaded in the resin could actually be quitelow, for example about 10% as illustrated in FIG. 2. The averagedistances, or length scales, between loaded HTC materials therefore maybe slightly greater than n, however, a large percentage will still beless than n and therefore fall within embodiments of the presentinvention. In particular embodiment, the percentage materials that areless than n distance from the next HTC material is over 50%, withparticular embodiment being over 75%. In particular embodiment theaverage length of the HTC materials is greater than n, which furtheraids in phonon transport.

The shorter n the greater the concentration of loaded HTC materials, andconversely, the greater the particle size, the less HTC materialsneeded. Particular embodiment use 5-60% loaded HTC materials by totalvolume of the resins and fillers, with more particular embodiments at25-40%. When the resin is impregnated into the tape, it will fill up thespaces between the tape fibers and substrates. The HTC distributionwithin the tape at this point, however, is often not optimized, and caneven have the mean distance between HTC materials greater than n.Practice of the present invention then compresses the resin impregnatedtapes and reduces the distances between the loaded HTC materials.

When a loaded resin is being impregnated into a tape, the fibers orparticles of the tape act to block some of the HTC materials,particularly if the resin is 30% or more filler. However, by compressingthe tapes, the reverse happens, and more fillers are trapped within thetape as the HTC materials attach themselves to non-mobile parts of theoverall structure. The HTC fillers even get pinned to one another. Inthe embodiments given, it has been implied that the fillers do not reactwith the resin matrix, however, in some embodiments the fillers do formcovalent bonds with the resin and form more homogeneous matrixes. In ahomogenous matrix, the resin molecules that are bound to fillers will beretained better than the unbound resin molecules during compression.

Resins are used in a plurality of industries, and have a large number ofuses. Different properties of the resins affect not only their uses, butalso the quality and efficiency of the products that they are used with.For example, when resins are used in electrical insulation applications,their characteristics of dielectric strength and voltage endurance needsto be high, as does the thermal stability and thermal endurance.However, often contrary to these objectives, resins usually will alsohave a low thermal conductivity. The present invention balances thevarious physical properties of resins and the insulation system they areintroduced into to produce a system that has a higher thermalconductivity than conventional electrically insulating materials whilemaintaining adequate, and even enhancing, key physical properties suchas dielectric strength, voltage endurance, thermal stability and thermalendurance, mechanical strength and viscoelastic response. Delaminationand microvoid formation resulting from stresses caused by thermal,vibration and mechanical cycling effects are reduced or eliminated. Asused herein, the term resin refers to all resins and epoxy resins,including modified epoxies, polyesters, polyurethanes, polyimides,polyesterimides, polyetherimides, bismaleimides, silicones,polysiloxanes, polybutadienes, cyanate esters, hydrocarbons etc. as wellas homogeneous blends of these resins. This definition of resinsincludes additives such as cross-linking agents, accelerators and othercatalysts and processing aids. Certain resins, such as liquid crystalthermosets (LCT) and 1,2 vinyl polybutadiene combine low molecularweights characteristics with good crosslinking properties. The resinscan be of an organic matrix, such as hydrocarbons with and withouthetero atoms, an inorganic matrix, containing silicate and/or aluminosilicate components, and a mixture of an organic and inorganic matrix.Examples of an organic matrix include polymers or reactive thermosettingresins, which if required can react with the reactive groups introducedon inorganic particle surfaces. Cross-linking agents can also be addedto the resins to manipulate the structure and segmental lengthdistribution of the final crosslinked network, which can have a positiveeffect on thermal conductivity. This thermal conductivity enhancementcan also be obtained through modifications by other resin additives,such as catalysts, accelerators and other processing aids. Certainresins, such as liquid crystal thermosets (LCT) and 1,2 vinylpolybutadiene combine low molecular weights characteristics with goodcrosslinking properties. These types of resins tend to conduct heatbetter because of enhanced micro and macro ordering of theirsub-structure which may lead to enhanced conduction of heat as a resultof improved phonon transport. The better the phonon transport, thebetter the heat transfer.

When the high thermal conductivity fillers of the present invention aremixed with resins they form a continuous product, in that there is nointerface between the resins and the fillers. In some cases, covalentbonds are formed between the fillers and the resin. However, continuousis somewhat subjective and depends on the scale to which the observer isusing. On the macro-scale the product is continuous, but on thenano-scale there can still be distinct phases between the fillers andthe resin network. Therefore, when referring high thermal conductivityfillers mixing with the resin, they form a continuous organic-inorganiccomposite, on the macro-scale, while on the micro-scale the same mixturecan be referred to as a hybrid.

As mentioned, filled resin may be used in the electrical generator fieldwithout the tapes to fulfill electrical insulation applications in therotating and static electrical equipment components. The use of highthermal conductivity materials in a generator is multiple. Within thestator coil there are component materials other than the groundwallwhich must have high thermal conductivity to optimize the design.Likewise other components associated with the coils to maximize heatremoval. Improvements to stator design dictate that improvements be madeto rotor design so that generator efficiency can by maximized.

It is important that the interface between the various inorganic andorganic components is made to be chemically and physically intimate toensure a high degree of physical continuity between the different phasesand to provide interfaces which are mechanically strong and not prone tofailure. This is especially important during the operation of theelectrical insulation embodiments discussed, such as the electricalinsulation systems for both high and low voltage applications. Anenhanced interface integrity would enable an enhanced power rating,higher voltage stressing, reduced insulation thickness and high heattransfer.

Surface treatments to fillers introduce a variety of surface functionalgroups that are capable of compatibilizing inorganic surface of thefiller with the organic resin matrix. Typical surface treatment is tointroduce surface functional groups is to treat a surface physically(e.g. silane solution on metal oxides) to give reactive groups. Theinterface between the particle surface, such as the HTC filler, surfaceand the silane layer would only be held by physical bonding, such aspolar attraction and H-bonds. Although the silane surface could reactwith a resin that it is mixed in, there is no true chemical bond formedbetween the particle surface and the silane, i.e. essentially unreactivecoupling. Even if the substrate surface was rich in OH groups, such ashydrated Alumina, that could potentially react with the silane, it isunlikely that significant chemical bonds will form. In the case of theHTC fillers discussed herein, there would be virtually no chemical bondformation.

In order to obtain functional groups that are chemically attached to theHTC material (particle) surface, the present invention uses reactivegrafting. Reactive grafting occurs when the functional groups arechemically attached to the nanoparticle surface by a reactive process,such as by chemical reaction. Other processes include those that areplasma and radiation (e.g. UV, gamma, electron, etc.) driven, whichrequire appropriate environments and may be done in a multi-stageprocess. In this manner, a strong chemical bond is produced between thenanoparticle surface and the functional group attached (e.g., OH, COOH,NH2 and vinyl); i.e. reactive coupling. This would be the definition ofa reactive functional graft, i.e., the chemical attachment of afunctional group directly onto the particle surface. These reactivegrafting procedures are high energy compared to the physical bonding ofthe prior art, and use, for example, non-equilibrium plasma methods,chemical vapor and physical vapor deposition, sputter ion plating, laserbeams, electron and ion beam evaporation methods to chemically modifythe surfaces of the more inert surfaces of the HTC material, producingchemically attached functional species (e.g. OH, COOH, NH2, vinyl) whichare then reacted with resin to produce a continuous HTC matrix.

Specific examples of this include treating boron nitride (BN)nanoparticles with an electron beam in the presence of water vapor toproduce reactive N—OH groups that subsequently can be reacted with anLCT epoxy resin. The nitrogen of the reactive group comes directly fromthe boron nitride particle and remains linked to the particle. Thereforethe formulation is:B—N—OHwhere the boron is part of the larger nanoparticle. The hydroxyl (OH)group can then react directly with the resin or even anotherintermediate function group. Another example is modifying the surface ofAluminum Nitride nanoparticles in a hydrogen-rich vapor to producesurface NH2 reactive groups which can subsequently be reacted with a LCTepoxy or polyimide resin. Still another specific example uses a plasmapolymerization procedure, with Silicon Carbide nanoparticles, to producesurface grafted vinyl groups which can then be reacted with a vinylmonomer or a polybutadiene resin.

In a distinct aspect, the present invention involves a less energeticreactive grafting procedure involving wet chemical techniques ofselective reactions between an HTC material and a surface functionalgroup produce homogenous LCT-epoxy polymers with oligomers containinggrafted nano-sized HTC-materials.

In one of these distinct aspects the present invention relates to amethod of making homogenous LCT-epoxy polymers with oligomers containinggrafted nano-sized HTC-materials (HTC-oligomers). The dielectricstrength of these polymers is at least 1.2 kV/mil. The steps of makingthese polymers include grafting at least one functionalized organicgroup onto a nano-sized HTC-material to produce an HTC-oligomer. TheHTC-oligomer is then reacted with at least one LCT-epoxy resin underconditions sufficient to form a uniform dispersion and an essentiallycomplete co-reactivity of the HTC-oligomer with the LCT-epoxy resin(s).This reaction forms an intermediate resin-like mixture that is thencured to produce the homogenous LCT-epoxy polymers with HTC-oligomers.

In this aspect of the invention, the amount of the HTC-oligomer to theamount LCT-epoxy resin comprises a ratio of between 1:4 and 3:1 byweight. In a more particular embodiment of the wet chemical graftingtechnique the HTC-oligomer portions of the homogenous LCT-epoxy polymerswith HTC-oligomers is 20-50% by weight.

Though there are a variety of methods for preparing LCT-epoxy resins, aparticular method is warming the sample at approximately 60° C. untilthe LCT-epoxy resin is clear. Likewise, when mixing the LCT-epoxy resinand the HTC-oligomer, one method is to warm to approximately 60° C.until clear. The nano-sized HTC-material can be one or more of alumina,silica and a metal oxide. In a more particular embodiment of the wetchemical grafting technique the metal oxide is magnesium oxide. Otherappropriate HTC-materials will be apparent to one of ordinary skill inthe art.

In another embodiment of the wet chemical grafting technique, thegrafting the functionalized organic group(s) onto the nano-sizedHTC-material is performed by either a silane grafting or a free radicalgrafting. In a more particular embodiment of the wet chemical graftingtechnique, the silane grafting involves reactants chosen from4-trimethoxysilyl tetra-hydrophthalic anhydride (TSPA) and3-methacryloxpropyl trimethoxy silane (MOTPS). In another particularembodiment of the wet chemical grafting technique, the free radicalgrafting involves the reactant ceric ammonium nitrate.

In another embodiment of the wet chemical grafting technique, the methodfurther comprises mixing at least one anhydriding agent with either orboth of the LCT-epoxy resin(s) and the HTC-oligomers, where thehomogenous LCT-epoxy polymers with HTC-oligomers are a homogenousLCT-epoxy anhydride polymers with HTC-oligomers.

In a particular embodiment of the wet chemical grafting technique theanhydriding agent is taken from the group consisting of1-methylhexahydrophthalic anhydride and 1-methyltetrahydrophthalicanhydride. In another particular embodiment of the wet chemical graftingtechnique the anhydriding agent is approximately 20-40% by weight of thehomogenous LCT-epoxy anhydride polymers with HTC-oligomers. In anotherembodiment of the wet chemical grafting technique, the method furthercomprises mixing at least one vinyl agent with either or both of theLCT-epoxy resin(s) and the HTC-oligomers, where the homogenous LCT-epoxypolymers with HTC-oligomers are a homogenous LCT-epoxy vinyl polymerswith HTC-oligomers.

In another aspect, the present invention provides for a method of makinghomogenous LCT-epoxy polymers with HTC-oligomers that have a dielectricstrength of at least 1.2 kV/mil, which is coated on at least oneelectrical insulator. This method involves the steps of grafting atleast one functionalized organic group onto a nano-sized HTC-material toproduce HTC-oligomers. The HTC-oligomers are then reacted with at leastone LCT-epoxy resin where an intermediate resin-like mixture is formed.This mixture is then warmed under sufficient conditions to form auniform dispersion and an essentially complete co-reactivity of theHTC-oligomers with the LCT-epoxy resin(s). The mixture is thenimpregnated onto the electrical insulator and cured to produce thehomogenous LCT-epoxy polymers with HTC-oligomers. In this aspect theamount of the HTC-oligomers to the at least one LCT-epoxy resincomprises a ratio of between 1:4 and 3:1 by weight.

In one embodiment of the wet chemical grafting technique the methodfurther comprises mixing at least one anhydriding agent with one or bothof the LCT-epoxy resin(s) and the HTC-oligomers, where the homogenousLCT-epoxy polymers with HTC-oligomers are a homogenous LCT-epoxyanhydride polymers with HTC-oligomers. In another embodiment of the wetchemical grafting technique the method further comprises mixing at leastone vinyl agent with one or both of the at least one LCT-epoxy resin(s)and the HTC-oligomers, where the homogenous LCT-epoxy polymers withHTC-oligomers are a homogenous LCT-epoxy vinyl polymers withHTC-oligomers.

In another aspect the present invention provides homogenous LCT-epoxypolymers with HTC-oligomers. This comprises at least one HTC-oligomersub-structure that contains at least one nano-sized HTC-material graftedthereto and at least one LCT-epoxy sub-structure, where the HTC-oligomersub-structure is organically bonded to the LCT-epoxy substructure Thethermal conductivity in the transverse direction is at least 0.50 W/mKand in the thickness direction is at least 0.99 W/mK in an environmentof 25° C. The homogenous LCT-epoxy polymers with HTC-oligomers has adielectric strength of at least 1.2 kV/mil, and is substantially free ofparticle wetting and micro-void formation. Further, approximately 20-75%by weight of the homogenous LCT-epoxy polymers with HTC-oligomers is theHTC-oligomer sub-structure.

Either the homogenous LCT-epoxy polymers with HTC-oligomers or thehomogenous LCT-epoxy anhydride/vinyl polymers with HTC-oligomers may beproduced as a coating on insulative materials, such as a mica/glassinsulating tape. HTC-oligomers as used herein refers to any oligomerwith grafted nano-sized high thermal conductivity (HTC) material,according to the present invention.

Though there is no intention to be limited to a specific type ofHTC-oligomer, or a specific method of synthesizing HTC-oligomers for thepurposes of reacting with LCT-epoxy resins, particular nano-sizedHTC-materials used may be alumina, silica, and metal oxides, includingmagnesium oxide and zinc oxide. Furthermore, these materials may betreated in a variety of different ways to produce even more variation ondifferent kinds of HTC-oligomers. Examples of these include metal (oralumina or silica) oxide HTC-oligomers with the basic structure of:

-   where X represents the HTC-material, and R represents an organic    functional group.

As mentioned, the HTC-materials may be chemically grafted with a polymerstructure by a variety of methods to produce the multitude ofHTC-oligomers possible. A particular example of this is free radicalgrafting, where a reactant such as ceric ammonium nitrate may be used.Another particular examples is silane grafting. In this examplereactants used to produce functional groups include 4-trimethoxysilyltetra-hydrophthalic anhydride (TSPA) and 3-methacryloxpropyl trimethoxysilane (MOTPS). If these reactants are used, an additional silica group,beyond what may be represented in the X group, will be present:

An alumina X group with a TSPA functional group would therefore be:

-   In all cases, the functional group, R, is then used to react with a    given substance to produce a desired product.

In one embodiment of the wet chemical grafting technique the functionalgroup reacts with the epoxy group of an LCT-epoxy resin to produce anLCT-epoxy with HTC-oligomers. However, before, concurrently or evenafter reacting the functional group with the LCT-epoxy group, thefunctional group may also react with other substance to improve thereaction with the LCT-epoxy and/or the final polymer structure. Forexample, an anhydride or a vinyl group or both may be added with theLCT-epoxy resin, in the producing of the HTC-oligomers, or when reactingthe HTC-oligomer with the LCT-epoxy resin. In such a reaction, the finalproduct would be an LCT-epoxy anhydride polymer with HTC-oligomers or anLCT-epoxy vinyl polymer with HTC-oligomers or even an LCT-epoxyanhydride-vinyl polymer with HTC-oligomers. It should be noted thatthough the HTC-oligomer may be formed using an anhydride containingreagent, the term anhydride used herein describes resins and polymers ofthe present invention that have had an additional anhydride reagentadded.

The following is a particular method of making a suitable HTC-oligomeras used in the synthesis of homogenous LCT-epoxy polymers with oligomerscontaining grafted nano-sized HTC-materials (LCT-epoxy polymers withHTC-oligomers):

-   Graft polymerization reactions were carried out in a round-bottomed,    three-neck flask fitted with a stirrer, a gas inlet tube and a    thermometer. 2.0 g of nano-size magnesium oxide was dispersed in    25 ml. of distilled water, and the grafting reaction was carried out    under nitrogen by bubbling the gas through the reaction mixture. The    required amount of initiator solution was then added (0.55 g of    ceric ammonium nitrate dissolved in 10 ml of 1N nitric acid,    followed by 6.0 ml of methyl methacrylate. The reaction was allowed    to proceed for 3 hours at 40° C. The grafted product was extracted    in a soxhlet extractor to remove the polymer.

Though the below examples use powdered HTC-oligomers, it will beapparent to one of ordinary skill in the art that the HTC-materials maybe delivered to the reaction in other forms, such as in solution.

The synthesis of LCT-epoxy polymers with HTC-oligomers according to thepresent invention may similarly be done by a variety of methods thatwill be apparent to one of ordinary skill in the art after review of theprocedures contained herein. A particular method, however, comprises:

Alumina-grafted-TSPA-oligomer (HTC-oligomer) (2.5 g) was ground to afine powder in a porcelain mortar. LCT-epoxy resin RSS-1407 (4.0 g) waswarmed to 60° C. in a small glass jar. The HTC-oligomer powder was addedto the resin and the mixture stirred for approximately 30 min. until thesolution was clear. 0.1 g of zinc naphthenate was added as a curecatalyst and mixed over an additional 5 min.. The liquid was then pouredinto a small aluminum dish and placed in an oven at 150° C. for fourhours to cure.

This reaction may be summarized as follows:

The two HTC-oligomers with an R functional group are reacted with abiphenol LCT-epoxy chain that contains n repeating biphenol units. Theresult is an LCT-epoxy polymer with cross-linked HTC-oligomers. TheHTC-oligomers particles become organically bonded to the LCT-epoxychain. Though this example uses biphenol LCT-epoxy, this reaction couldbe performed with any variety of LCT, alone or in combination. Examplesof other LCTs can be found in U.S. Pat. No. 5,904,984, which isincorporated herein by reference.

The synthesis of the LCT-epoxy anhydride polymers with HTC-oligomers bythis example produces polymers with approximately 38% by weightHTC-oligomer. The remaining percentage is primarily the LCT-epoxies witha small amount of accelerants and other materials. Though this is oneembodiment of the wet chemical grafting technique of the presentinvention, the HTC-oligomer content may be anywhere from approximately20-75% by weight. With a particular embodiment of the wet chemicalgrafting technique being from 30-55% by weight and an even more specificembodiment of the wet chemical grafting technique of 35-50% by weight.

Similar to the synthesis of the LCT-epoxy polymers with HTC-oligomers,an example of the synthesis of an LCT-epoxy-anhydride polymers withHTC-oligomers comprises:

Biphenol LCT-epoxy resin RSS-1407 (4.0 g) was added to1-methylHexahydrophthalic anhydride (4.0 g) stirring in a small glassjar warmed to 60° C. on a hot plate. After the solution was clear, analumina-grafted-TSPA-oligomer (HTC-oligomer) (3.0 g) was added and thesolution stirred further at 60° C. until the solution was again clear.0.1 g of zinc naphthenate was added as a cure accelerator and mixed overan additional 5 min. The liquid was then poured into a small aluminumdish and placed in an oven at 150° C. for four hours to cure.

The use of the anhydride components adds additional reactivity to thisreaction, aiding the HTC-oligomer's co-reactivity with the LCT-epoxies.Further, the resulting polymers are more fluid, with improved insulativeproperties. In this example the anhydrides make up approximately 36% byweight of the final LCT-epoxy-anhydride polymers. Though this is oneembodiment of the wet chemical grafting technique of the presentinvention, the anhydride content may be anywhere from approximately20-40% by weight. In this example, the overall percentage ofHTC-oligomers is lower than that of the above example. This might notalways be the case, and the addition of anhydride might not reduce theoverall percentage of HTC-materials in the resulting polymers.

In both of the above examples, a LCT-epoxy polymer with HTC-oligomersmay also contain a vinyl group. A variety of methods for including avinyl group would be apparent to one of ordinary skill the art. However,a particular method of making an LCT-epoxy vinyl polymer withHTC-oligomers, or an LCT-epoxy anhydride-vinyl polymer withHTC-oligomers, would be to follow the above examples, but begin with aMOTPS-oligomer instead of a TSPA-oligomer. In following with the aboveexamples, when the cure accelerator is added, add a vinyl containingreactant, such as the di-functional monomer, p-vinylphenylglycidylether(which, in keeping with the above sample sized, would be approximately1.0 g).

The addition of a vinyl group to the reaction is dependent upon whattypes of reagents are being used and under what conditions. For example,some LCT-epoxy resins contain styrene. Therefore the vinyl group wouldallow for a more complete reaction of the LCT-epoxy resin and theHTC-oligomers, therefore producing a better and more homogeneouspolymer. If a vinyl group is added, its approximate percentage in thefinal polymer will 4-16% by weight.

One embodiment of the present invention adds high thermal conductivity(HTC) materials to resins to improve the thermal conductivity of theresins. In some embodiments the other physical properties of the resinsare reduced in a trade-off with higher thermal conductivity, but inother embodiments, some of the other physical properties will not besignificantly affected, and in some particular embodiments these otherproperties will be improved. In particular embodiments, the HTCmaterials are added to resins, such as LCT epoxy, that have orderedsub-structures. When added to these types of resins, the amount of HTCmaterial used can be reduced versus use in resins without orderedsub-structures.

The HTC materials loaded into the resins are of a variety of substancesthat can be added so that they may physically and/or chemically interactwith or react with the resins to improve thermal conductivity. In oneembodiment, the HTC materials are dendrimers, and in another embodimentthey are nano or micro inorganic fillers having a defined size or shapeincluding high aspect ratio particles with aspect ratios (ratio meanlateral dimension to mean longitudinal dimension) of 3 to 100 or more,with a more particular range of 10-50.

In a related embodiment, the HTC materials may have a defined size andshape distribution. In both cases the concentration and relativeconcentration of the filler particles is chosen to enable a bulkconnecting (or so-called percolation) structure to be achieved whichconfers high thermal conductivity with and without volume filling toachieve a structurally stable discrete two phase composite with enhancedthermal conductivity. In another related embodiment, the orientation ofthe HTC materials increases thermal conductivity. In still anotherembodiment, the surface coating of the HTC materials enhances phonontransport. These embodiments may stand apart from other embodiments, orbe integrally related. For example, dendrimers are combined with othertypes of highly structured materials such as thermoset and thermoplasticmaterials. They are uniformly distributed through a resin matrix suchthat the HTC materials reduce phonon scattering and provide micro-scalebridges for phonons to produce good thermally conducting interfacesbetween the HTC materials. The highly structured materials are alignedso that thermal conductivity is increased along a single direction ordirections to produce either localized or bulk anisotropic electricallyinsulating materials. In another embodiment HTC is achieved by surfacecoating of lower thermal conductivity fillers with metal oxides,carbides or nitrides and mixed systems having high thermal conductivitywhich are physically or chemically attached to fillers having definedbulk properties, such attachment being achieved by processes such aschemical vapour deposition and physical vapour deposition and also byplasma treatment.

In related embodiments, the HTC materials form essentially homogenousmixtures with the resins, essentially free of undesired microscopicinterfaces, variable particle wetting and micro void formation. Thesehomogeneous materials form a continuous-phase material which arenon-discrete at length scales shorter than either the phonon wavelengthor phonon mean free path in conventional electrical insulatingmaterials. In some embodiments, intentional interfaces can be placed inthe resin structure so as to control dielectric breakdown. In insulatingmaterials, dielectric breakdown will occur given the right conditions.By controlling the nature and spatial distribution of the interfaces intwo-phase system, dielectric breakdown strength and long term electricalendurance can be enhanced. Increases in dielectric strength will takeplace in part because of increased densification, the removal of microvoids and a higher level of internal mechanical compression strength.

Resins of the present invention may be used for impregnation of othercomposite constructions such as a mica tape and glass and polyestertape. In addition to the standard mica (Muscovite,Phlogopite) that istypically used for electrical insulation there is also Biotite mica aswell as several other mica-like Alumino-Silicate materials such asKaolinite, Halloysite, Montmorillonite and Chlorite. Montmorillonite haslattices in its structure which can be readily intercalated with polymerresins, metal cations and nano particles to give high dielectricstrength composites.

In other embodiments, the present invention is used as a continuouscoating on surfaces where insulation is desired; note that “continuouscoating” is a description of a macro-scale application. In a continuouscoating, the resin forms a coating on materials without the need for atape or other substrate. When used with a substrate, the HTC materialscan be combined with the resin by a variety of different methods. Forexample, they can be added prior to the resin being added to thesubstrate, or the HTC materials can be added to the substrate before theresin is impregnated thereon, or the resin can be added first, followedby the HTC material and then an additional impregnation of resin. Otherfabrication and process methods will be apparent to one of ordinaryskill in the art.

In one embodiment the present invention uses novel organic-inorganicmaterials which offer higher thermal conductivity and also maintain orenhance other key properties and performance characteristics. Suchmaterials have applications in other high voltage and low voltageelectrical insulation situations where high thermal conductivity confersadvantage in terms of enhanced power rating, reduced insulationthickness, more compact electrical designs and high heat transfer. Thepresent invention adds nano, meso, and micro inorganic HTC-materialssuch as alumina, magnesium oxide, silicon carbide, boron nitride,aluminium nitride, zinc oxide and diamond, as well as others, to givehigher thermal conductivity. These materials can have a variety ofcrystallographic and morphological forms and they may be processed withthe matrix materials either directly or via a solvent which acts as acarrier liquid. The solvent mixture may be used to mix the HTC-materialsinto the matrix to various substrates such as mica-tape. In contrast,molecular hybrid materials which form another embodiment of the presentinvention, do not contain discrete interfaces, and have the advantagesconferred by an inorganic phase within an organic. These materials mayalso confer enhancement to other physical properties such as thermalstability, tensile strength, flexural strength, and impact strength,variable frequency and temperature dependant mechanical moduli and lossand general viscoelastic response, etc.

In another embodiment, the present invention comprises discreteorganic-dendrimer composites in which the organic-inorganic interface isnon-discrete with a dendrimer core-shell structure. Dendrimers are aclass of three-dimensional nanoscale, core-shell structures that buildon a central core. The core may be of an organic or inorganic material.By building on a central core, the dendrimers are formed by a sequentialaddition of concentric shells. The shells comprise branched moleculargroups, and each branched shell is referred to as a generation.Typically, the number of generations used is from 1-10, and the numberof molecular groups in the outer shell increase exponentially with thegeneration. The composition of the molecular groups can be preciselysynthesized and the outer groupings may be reactive functional groups.Dendrimers are capable of linking with a resin matrix, as well as witheach other. Therefore, they may be added to a resin as an HTC material,or, in other embodiments, may form the matrix themselves without beingadded to traditional resins.

The molecular groups can be chosen for their ability to react, eitherwith each other or with a resin. However, in other embodiments, the corestructure of the dendrimers will be selected for their own ability toaid in thermal conductivity; for example, metal oxides as discussedbelow.

Generally, the larger the dendrimer, the greater its ability to functionas a phonon transport element. However, its ability to permeate thematerial and its percolation potential can be adversely affected by itssize so optimal sizes are sought to achieve the balance of structure andproperties required. Like other HTC materials, solvents can be added tothe dendrimers so as to aid in their impregnation of a substrate, suchas a mica or a glass tape. In many embodiments, dendrimers will be usedwith a variety of generations with a variety of different moleculargroups.

Commercially available organic Dendrimer polymers includePolyamido-amine Dendrimers (PAMAM) and Polypropylene-imine Dendrimers(PPI) and PAMAM-OS which is a dendrimer with a PAMAM interior structureand organo-silicon exterior. The former two are available from AldrichChemical™ and the last one from Dow-Corning™.

Similar requirements exist for inorganic-organic dendrimers which may bereacted together or with matrix polymers or reactive resins to form asingle material. In this case the surface of the dendrimer could containreactive groups similar to those specified above which will either allowdendrimer-dendrimer, dendrimer-organic, dendrimer-hybrid, anddendrimer-HTC matrix reactions to occur. In this case the dendrimer willhave an inorganic core and an organic shell, or vice-versa containingeither organic or inorganic reactive groups or ligands of interest. Itis therefore also possible to have an organic core with an inorganicshell which also contains reactive groups such as hydroxyl, silanol,vinyl-silane, epoxy-silane and other groupings which can participate ininorganic reactions similar to those involved in common sol-gelchemistries.

In all cases phonon transport is enhanced and phonon scattering reducedby ensuring the length scales of the structural elements are shorterthan or commensurate with the phonon distribution responsible forthermal transport. Larger HTC particulate materials can actuallyincrease phonon transport in their own right, however, smaller HTCmaterials can alter the nature of the resin matrix, thereby affect achange on the phonon scattering. This may be further assisted by usingnano-particles whose matrices are known to exhibit high thermalconductivity and to ensure that the particle size and interfacialcharacteristics are sufficient to sustain this effect and also tosatisfy the length scale requirements for reduced phonon scattering. Itis also necessary to consider the choice of structures that are morehighly ordered including reacted dendrimer lattices having both shortand longer range periodicity and ladder or ordered network structuresthat may be formed from matrices such as liquid crystal epoxy resins andpolybutadienes. A resin matrix of the prior art will have a maximumthermal conductivity of about 0.15 W/mK. The present invention providesresins with a thermal conductivity of 0.5 to 5 W/mK and even greater.

Continuous organic-inorganic hybrids may be formed by incorporatinginorganic nano-particles in linear or crosslinked polymers andthermosetting resins in which nano-particles dimensions are of the orderof or less than the polymer or network segmental length (typically 1 to50 nm). This would include, but is not exclusive to three routes ormechanisms by which this can occur (i) side chain grafting, (ii)inclusive grafting e.g. between two polymer chain ends, (iii) cross-linkgrafting involving at least two and typically several polymer molecules.These inorganic nano-particles will contain reactive surfaces to formintimate covalently bonded hybrid organic-inorganic homogeneousmaterials. These nano-particles may be metal oxides, metal nitrides, andmetal carbides, as well as some non-metal oxides, nitrides and carbides.For example, alumina, magnesium oxide and zinc oxide and other metaloxides, boron nitride and aluminum nitride and other metal nitrides,silicon carbide and other carbides, diamond of natural or syntheticorigin, and any of the various physical forms of each type and othermetal carbides and hybrid stoichiometric and non-stoichiometric mixedoxides, nitrides and carbides. More specific examples of these includeAl₂O₃, AlN, MgO, ZnO, BeO, BN, Si₃N₄, SiC and SiO₂ with mixedstoichiometric and non-stoichiometric combinations. Further, thesenano-particles will be surface treated to introduce a variety of surfacefunctional groups which are capable of participating in reactions withthe host organic polymer or network. It is also possible to coat non-HTCmaterials, such as silica and other bulk filler materials, with an HTCmaterial. This may be an option when more expensive HTC materials areused.

The volume percentage of the HTC materials in the resin may be up toapproximately 60% or more by volume, and more particularly up toapproximately 35% by volume. Higher volume filling tends to give higherstructural stability to a matrix. However, with control of the size andshape distribution, degree of particle association and alignment the HTCmaterials can occupy as little as 1% by volume or less. Although, forstructural stability reasons, it might be useful to add an amountgreater than the minimum needed for percolation to occur. Therefore theresin can withstand physical strains and deformation without damagingthe percolation structure and the HTC characteristics.

The addition of surface functional groups may include hydroxyl,carboxylic, amine, epoxide, silane or vinyl groups which will beavailable for chemical reaction with the host organic polymer or networkforming resin system. These functional groups may be naturally presenton the surface of inorganic fillers or they may be applied using wetchemical methods, non-equilibrium plasma deposition including plasmapolymerization, chemical vapour and physical vapour deposition, sputterion plating and electron and ion beam evaporation methods. The matrixpolymer or reactive resin may be any system which is compatible with thenano-particles and, if required, is able to react with the reactivegroups introduced at the nano-particle surface. These may be epoxy,polyimide epoxy, liquid crystal epoxy, cyanate-ester and other lowmolecular weight polymers and resins with a variety of crosslinkingagents.

In the case of non-discrete organic-inorganic hybrids it is possible touse sol-gel chemistry to form a continuous molecular alloy. In this casesol-gel chemistries involving aqueous and non-aqueous reactions may beconsidered.

The products of the present invention exhibit higher thermalconductivity than conventional electrically insulating materials and maybe used as bonding resins in mica-glass tape constructions, as unreactedvacuum-pressure impregnation resins for conventional mica tapeconstructions and as stand alone materials to fulfill electricalinsulation applications in rotating and static electrical power plantand in both high and low voltage electrical and electronic equipment,components and products. Products of the present invention may becombined with each other, as well as HTC-material, and other materials,of the prior art.

Micro and nano HTC particles may be selected on their ability to selfaggregate into desired structural forms such as filaments and brancheddendrites. Particles may be selected for their ability to self-assemblenaturally, though this process may also be modified by external forcessuch as an electric field, magnetic field, sonics, ultra-sonics, pHcontrol, use of surfactants and other methods to affect a change to theparticle surface charge state, including charge distribution, of theparticle. In a particular embodiment, particles such as boron nitride,aluminum nitride, diamond are made to self assemble into desired shapes.In this manner, the desired aggregation structures can be made fromhighly thermally conductive materials at the outset or assembled duringincorporation into the host matrix.

In many embodiments, the size and shape of the HTC-materials are variedwithin the same use. Ranges of size and shape are used in the sameproduct. A variety of long and shorter variable aspect ratioHTC-materials will enhance the thermal conductivity of a resin matrix,as well as potentially provide enhanced physical properties andperformance. One aspect that should be observed, however, is that theparticle length does not get so long as to cause bridging between layersof substrate/insulation. Also, a variety of shapes and length willimprove the percolation stability of the HTC-materials by providing amore uniform volume filing and packing density, resulting in a morehomogeneous matrix. When mixing size and shapes, in one embodiment thelonger particles are more rod-shaped, while the smaller particles aremore spheroidal, platelet or discoid and even cuboids. For example aresin containing HTC-materials could contain about 55-65% by volume10-50 nm diameter spheroids and about 15-25% by volume 10-50 μm lengthrods, with 10-30% volume resin.

In another embodiment the present invention provides for new electricalinsulation materials based on organic-inorganic composites. The thermalconductivity is optimized without detrimentally affecting otherinsulation properties such as dielectric properties (permittivity anddielectric loss), electrical conductivity, electric strength and voltageendurance, thermal stability, tensile modulus, flexural modulus, impactstrength and thermal endurance in addition to other factors such asviscoelastic characteristics and coefficient of thermal expansion, andoverall insulation. Organic and inorganic phases are constructed and areselected to achieve an appropriate balance of properties andperformance.

In one embodiment the surface coating of nano, meso and micro inorganicfillers having the desired shape and size distribution and the selectedsurface characteristics and bulk filler properties are complimentary toeach other. This enables the percolation structure of the filler phasein the organic host and the interconnection properties to be controlledindependently while maintaining required bulk properties. In additionorganic and inorganic coatings, as singular or secondary coatings may beused to ensure compatibilisation of the particle surfaces with theorganic matrix and allow chemical reactions to occur with the hostorganic matrix.

In regards to shape, the present invention utilizes individual particleshapes tending towards natural rods and platelets for enhancedpercolation, with rods being the most preferred embodiment includingsynthetically processed materials in addition to those naturally formed.A rod is defined as a particle with a mean aspect ratio of approximately5 or greater, with particular embodiments of 10 or greater, though withmore particular embodiments of no greater than 100. In one embodiment,the axial length of the rods is approximately in the range 10 nm to 100microns. Smaller rods will percolate a resin matrix better, and haveless adverse effect on the viscosity of the resin.

Many micro and nano particles form spheroidal and discoid shapes, whichhave reduced ability to distribute evenly under certain conditions andso may lead to aggregated filamentary structures that reduce theconcentration at which percolation occurs. By increasing thepercolation, the thermal properties of the resin can be increased, oralternately, the amount of HTC material that needs to be added to theresin can be reduced. Also, the enhanced percolation results in a moreeven distribution of the HTC materials within the resin rather thanagglomeration which is to be avoided, creating a more homogenous productthat is less likely to have undesired interfaces, incomplete particlewetting and micro-void formation. Likewise aggregated filamentary ordendritic structures, rather than globular (dense) aggregates oragglomerates, formed from higher aspect ratio particles confer enhancedthermal conductivity.

Additionally, fluid flow fields and electric and magnetic fields can beapplied to the HTC materials to distribute and structurally organizethem inside of the epoxy resin. By using alternating or static electricfields, the rod and platelet shapes can be aligned on a micro-scale.This creates a material that has different thermal properties indifferent directions. The creation of an electric field may beaccomplished by a variety of techniques known in the art, such as byattaching electrodes across an insulated electrical conductor or by useof a conductor in the centre of a material or the insulation system.

Organic surface coatings, and inorganic surface coatings such as,metal-oxide, -nitride, -carbide and mixed systems may be generatedwhich, when combined with the selected particle size and shapedistribution, provide a defined percolation structure with control ofthe bulk thermal and electrical conductivity of the insulation systemwhile the particle permittivity may be chosen to control thepermittivity of the system. Another type of coating is micro-particulateand nano-particulate diamond coatings and of natural or syntheticorigin. In poly-crystalline and mono-crystalline nano-particulate form,the particles may associate with the surface of a carrier particle, egsilica. Silica by itself is not a strong thermally conducting material,but with the addition of a surface coating it becomes more of a higherthermal conductivity material. Silica and other such materials, however,have beneficial properties such as being readily formed into rod-shapedparticles, as discussed above. In this manner, various HTC propertiescan be combined into one product. These coatings may also haveapplication to mica tape structures, including both the mica and theglass components, with or without resin impregnation.

Reactive surface functional groups may be formed from surface groupsintrinsic to the inorganic coating or may be achieved by applyingadditional organic coatings both of which may include hydroxyl,carboxylic, amine, epoxide, silane, vinyl and other groups which will beavailable for chemical reaction with the host organic matrix. Thesesingle or multiple surface coatings and the surface functional groupsmay be applied using wet chemical methods, non-equilibrium plasmamethods including plasma polymerization and chemical vapour and physicalvapour deposition, sputter ion plating and electron and ion beamevaporation methods.

In another embodiment the present invention provides for new electricalinsulation systems based on organic-inorganic composites. The interfacebetween the various inorganic and organic components is made to bechemically and physically intimate to ensure a high degree of physicalcontinuity between the different phases and to provide interfaces whichare mechanically strong and not prone to failure during the operation ofthe electrical insulation system in service in both high and low voltageapplications. Such materials have applications in high voltage and lowvoltage electrical insulation situations where enhanced interfacialintegrity would confer advantage in terms of enhanced power rating,higher voltage stressing of the insulation systems, reduced insulationthickness and would achieve high heat transfer.

A particular embodiment uses a variety of surface treatments, nano, mesoand micro inorganic fillers, so as to introduce a variety of surfacefunctional groups which are capable of compatibilizing the inorganicsurface with respect to the organic matrix or to allow chemicalreactions to occur with the host organic matrix. These surfacefunctional groups may include hydroxyl, carboxylic, amine, epoxide,silane or vinyl groups which will be available for chemical reactionwith the host organic matrix. These functional groups may be appliedusing wet chemical methods, non-equilibrium plasma methods, chemicalvapour and physical vapour deposition, sputter ion plating and electronand ion beam evaporation methods.

In many embodiments, the surface treated materials may be used inbonding resins in mica-glass tape constructions, in unreactedvacuum-pressure impregnation (GVPI & VPI) resins for conventional micatape constructions and in stand alone electrical insulation coatings orbulk materials to fulfill either electrically insulating or conductingapplications in rotating and static electrical power plant and in bothhigh and low voltage electrical equipment, components and products.Also, all chemical reactions should be the result of addition, and notcondensation reactions so as to avoid volatile by-products.

Improvements in epoxy resins have recently been made by using liquidcrystal polymers. By mixing an epoxy resin with a liquid crystal monomeror by incorporating a liquid crystalline mesogen into an epoxy resinmolecule such as DGEBA, a liquid crystal thermoset (LCT) epoxy resin isproduced that contains polymers or monomers that can be cross-linked toform ordered networks having significantly improved mechanicalproperties. See U.S. Pat. No. 5,904,984, which is incorporated herein byreference. A further benefit of LCTs is that they also have improvedthermal conductivity over standard epoxy resins, and lower coefficientof thermal expansion (CTE) values as well.

What makes LCT epoxy resins even more appealing is that they are alsobetter able to conduct heat than a standard epoxy resin. U.S. Pat. No.6,261,481, which is incorporated herein by reference, teaches that LCTepoxy resins can be produced with a thermal conductivity greater thanthat of conventional epoxy resins. For example, a standard Bisphenol Aepoxy is shown to have thermal conductivity values of 0.18 to 0.24 wattsper meter degree Kelvin (W/mK) in both the transverse (plane) andthickness direction. By contrast, an LCT epoxy resin is shown to have athermal conductivity value, when used in practical applications, of nomore than 0.4 W/mK in the transverse direction and up to 0.9 W/mK in thethickness direction.

As used in reference to HTC materials being applied to paper, the termsubstrate refers to the host material that the insulating paper isformed from, while paper matrix refers to the more complete papercomponent made out of the substrate. These two terms may be usedsomewhat interchangeable when discussing this embodiment of the presentinvention. The increase of thermal conductivity should be accomplishedwithout significantly impairing the electrical properties, such asdissipation factor, or the physical properties of the substrate, such astensile strength and cohesive properties. The physical properties caneven be improved in some embodiments, such as with surface coatings. Inaddition, in some embodiments the electrical resistivity of the hostpaper matrix can also be enhanced by the addition of HTC materials.

In addition to the standard mica (Muscovite, Phlogopite) that istypically used for electrical insulation there is also Biotite mica aswell as several other Mica-like Alumino-Silicate materials such asKaolinite, Halloysite, Montmorillonite and Chlorite. Montmorillonite haslattices in its structure which can be readily intercalated with HTCmaterials such as metal cations, organic compounds and monomers andpolymers to give high dielectric strength composites.

Insulating papers are just one type of porous media that may beimpregnated with the resin of the present invention. Many othermaterials and components made therefrom, in many industries, some ofwhich are mentioned below, can use different types of porous media toimpregnate the resin into. By way of examples there are glass fibermatrices or fabric, and polymer matrices or fabric, where the fabricmight typically be cloth, matt, or felt. Circuit boards, which are glassfabric laminate, with planar lamination, will be one product which willbenefit from the use of resins of the present invention.

Types of resin impregnation used with stator coils are known as VPI andGVPI. Tape is wrapped around the coil and then impregnated with lowviscosity liquid insulation resin by vacuum-pressure impregnation (VPI).That process consists of evacuating a chamber containing the coil inorder to remove air and moisture trapped in the mica tape, thenintroducing the insulation resin under pressure to impregnate the micatape completely with resin thus eliminating voids, producing resinousinsulation in a mica host. A compression of about 20% is particular tothe VPI process in some embodiments. After this is completed, the coilsare heated to cure the resin. The resin may contain an accelerator orthe tape may have one in it. A variation of this, global VPI (GVPI)involves the process where dry insulated coils are wound, and then thewhole stator is vacuum pressure impregnated rather than the individualcoils. In the GVPI process, the coils are compressed prior toimpregnation with the resin since the dry coils are inserted into theirfinal position prior to impregnation. Although various compressionmethods have been discussed above, it is also possible to use theVPI/GVPI impregnating process for the actual compression stage of thepresent invention.

In a particular embodiment the present invention provides for continuoushigh thermal conductivity resin that comprises a host resin matrix and ahigh thermal conductivity filler. The high thermal conductivity fillerforms a continuous organic-inorganic composite with the host resinmatrix via surface functional groups that are grafted to the highthermal conductivity filler and forms covalent linkages with the hostresin matrix. In a related embodiment the high thermal conductivityfillers are from 1-1000 nm in length, and have aspect ratios of between3-100. More particular aspect ratios are between 10-50.

In another related embodiment the high thermal conductivity fillers arechosen from one or more of oxides, nitrides, carbides and diamond. Whilethe surface functional groups are chosen from one or more of hydroxyl,carboxylic, amine, epoxide, silane and vinyl groups.

In another particular embodiment the present invention provides forcontinuous organic-inorganic resin with grafted functional groupsbridging the organic-inorganic boundary that comprises a host resinnetwork and inorganic high thermal conductivity fillers evenly dispersedin the host resin network and essentially completely co-reacted with thehost resin network. The high thermal conductivity fillers have a lengthof between 1-1000 nm and aspect ratios of 10-50. The high thermalconductivity fillers are selected from one or more of oxides, nitrides,and carbides and the continuous organic-inorganic resin comprises amaximum of 60% by volume of the high thermal conductivity fillers, andin other embodiments a maximum of 35%. Particularly, the high thermalconductivity fillers have surface functional groups that are grafted tothe high thermal conductivity fillers and the surface functional groupsallow for the essentially complete co-reactivity with the host resinnetwork.

In related embodiments the functional groups comprise one or more ofhydroxyl, carboxylic, amine, epoxide, silane and vinyl groups. The oneor more of oxides, nitrides, and carbides comprise Al2O3, AlN, MgO, ZnO,BeO, BN, Si3N4, SiC and SiO2 with mixed stoichiometric andnon-stoichiometric combinations. The host resin network includes epoxy,polyimide epoxy, polyimide, liquid crystal epoxy, polybutadiene,polyester and cyanate-ester. The continuous organic-inorganic resinfurther can also comprises a cross-linking agent, and the entire resincan be impregnated into a porous media.

In still another particular embodiment the present invention providesfor method of making a high thermal conductivity resin that comprisessupplying a host resin matrix and gathering a high thermal conductivitymaterial, which is then surface treated with reactive surface functionalgroups in a high energy reaction such that the surface functional groupsbecome grafted to the high thermal conductivity materials. Then mixingthe treated high thermal conductivity materials with the host resinmatrix such that the high thermal conductivity materials aresubstantially uniformly dispersed within the host resin matrix, and thenreacting the surface functional groups that are grafted to the highthermal conductivity materials with the host resin matrix to produce thehigh thermal conductivity resin. The amount of the high thermalconductivity materials in the high thermal conductivity resin is amaximum of 60% by volume, and the high energy reaction produces bondstrength of between approximately 200-500 kJ/mol.

In a related embodiment the method further comprises inserting across-linking agent. In other related embodiments the surface functionalgroups comprise one or more of hydroxyl, carboxylic, amine, epoxide,silane and vinyl groups, and the high energy reaction comprises one ofnon-equilibrium plasma radiation, chemical vapor and physical vapordeposition, sputter ion plating, laser beams, electron and ion beamevaporation.

In another specific embodiment the high thermal conductivity materialcomprises one or more of diamonds, AlN, BN, Si3N4, and SiC. This groupof HTC materials is particularly suited for forming bond strengths ofbetween approximately 200-500 kJ/mol, since the internal bonds of thematerials are very strong.

Although the present invention has been discussed primarily in use withelectrical industries, the invention is equally applicable in otherareas. Industries that need to increase heat transference would equallybenefit from the present invention. For example, the energy, chemical,process and manufacturing industries, inclusive of oil and gas, and theautomotive and aerospace industries. Other focuses of the presentinvention include power electronics, conventional electronics, andintegrated circuits where the increasing requirement for enhanceddensity of components leads to the need to remove heat efficiently inlocal and large areas. Also, while specific embodiments of the inventionhave been described in detail, it will be appreciated by those skilledin the art that various modifications and alternatives to those detailscould be developed in light of the overall teachings of the disclosure.Accordingly, the particular arrangements disclosed are meant to beillustrative only and not limiting as to the scope of the inventionswhich, is to be given the full breadth of the claims appended and anyand all equivalents thereof.

1. A continuous high thermal conductivity resin comprising: a host resinmatrix; and a plurality of nanosized high thermal conductivity fillersconsisting essentially of fillers having a length of from about 1-1000nm; wherein said plurality of nanosized high thermal conductivityfillers form a continuous organic-inorganic composite with said hostresin matrix via surface functional groups that are grafted to saidplurality of nanosized high thermal conductivity fillers and formcovalent linkages with said host resin matrix; wherein said continuousorganic-inorganic composite is effective to reduce phonon scattering andincrease phonon transport in the continuous high thermal conductivityresin while at least maintaining dielectric strength and voltageendurance of the continuous high thermal conductivity resin; wherein thecontinuous high thermal conductivity resin has a viscosity that rendersthe continuous high thermal conductivity resin suitable for impregnationby vacuum-pressure impregnation (VPI) or global vacuum-pressureimpregnation (GVPI); wherein said host resin matrix comprises an epoxyresin; wherein said surface functional groups grafted to said pluralityof nanosized high thermal conductivity fillers comprise epoxide groups;and wherein said epoxide groups grafted to said plurality of nanosizedhigh thermal conductivity fillers react directly with the epoxy resin toform said continuous organic-inorganic composite.
 2. The resin of claim1, wherein said plurality of nanosized high thermal conductivity fillersare chosen from at least one of oxides, nitrides, carbides or diamond.3. The resin of claim 1, wherein said plurality of nanosized highthermal conductivity fillers have an aspect ratio of between 3-100. 4.The resin of claim 3, wherein said plurality of nanosized high thermalconductivity fillers have an aspect ratio of between 10-50.
 5. Animpregnated porous media comprising an insulating paper impregnated withthe continuous high thermal conductivity resin of claim 1, wherein theimpregnated resin comprises minimal to no voids formed therein.
 6. Theresin of claim 1, wherein said surface functional groups furthercomprise at least one of hydroxyl, carboxylic, amine, silane or vinylgroups.
 7. The continuous high thermal conductivity resin of claim 1,wherein at least 75% of said plurality of nanosized high thermalconductivity fillers comprise a filler having a mean path length betweenitself and an adjacent second filler that is less than a phonon meanfree path of the host resin matrix.
 8. The continuous high thermalconductivity resin of claim 1, wherein said continuous organic-inorganiccomposite comprises a maximum of 35% by volume of the nanosized highthermal conductivity fillers.
 9. The continuous high thermalconductivity resin of claim 1, wherein said functional groups arechemically and directly grafted to said plurality of nanosized highthermal conductivity fillers.